This invention relates to a method for monitoring the system pressure of a continuously variable transmission (CVT) for motor vehicles which is connected on one side, via a permanent mechanical connection, with the prime mover of a motor vehicle and, on the other side, via an output line, with the gears of a motor vehicle by means of the electronic control.
Continuously variable transmissions, also called CVTs, have a variator for ratio adjustment; known structural shapes are toroid and belt-drive variators, for example. A belt-drive variator has a first cone pulley pair upon an input shaft and a second cone pulley pair upon an output shaft. Each cone pulley pair consists of a first pulley stationary in an axial direction and a second cone pulley movable in an axial direction, also respectively called primary pulley and secondary pulley. A belt-drive organ, such as a pushing link band, rotates between the cone pulley pairs. The primary pulley and secondary pulley are adjusted by a pressure medium. To this end, the electronic transmission control regulates the pressure level of the adjustment spaces of the primary pulley and secondary pulley, via electromagnetic actuators and hydraulic valves.
The orderly operation of a CVT is often monitored by the electronic transmission control. It is known from EP B 228 884 to monitor the orderly operation of two rotational speed sensors which are associated with the primary and secondary cone pulley pairs, it being tested whether or not the rotational speed signal is present. In the absence of the rotational speed of the secondary cone pulley pair, the reduction ratio can be controlled by the electronic transmission control to a fail-safe value. In case of absence of the rotational speed of the primary cone pulley pair, the rotational speed of the primary cone pulley pair is also controlled to a fail-safe value.
From DE A 44 36 506, a device for control of a CVT is further known which, via an input shaft, is connected with an input unit which can be a brake prime mover or an electromotor. The CVT has a hydrodynamic converter with lock-up clutch, a reversing set, a first cone pulley pair, a second cone pulley pair, a belt-drive organ and a hydraulic control device. The input shaft drives the hydrodynamic converter which consists of an impeller, a turbine wheel and a stator. A lock-up clutch is associated with the hydrodynamic converter. The turbine wheel and the converter lock-up clutch are connected with a transmission input shaft. The input shaft of the transmission drives the reversing gear set through which the rotational speed of the transmission input shaft can be directly transmitted to the first cone pulley pair or the direction of rotation can be reversed for reverse gear.
The CVT is controlled by the electronic transmission control by means of electromagnetic actuators and hydraulic valves which regulate the clutches and brakes and the respective pressure curves thereof. The electromagnetic actuators and hydraulic valves are relocated in the hydraulic valves which regulate the clutches and brakes and the respective pressure curves thereof. The electromagnetic actuators and hydraulic valves are located in the hydraulic control devices; a multiplicity of monitoring signals are fed to the electronic transmission control, for example, the load position of the input unit, the rotational speed of the transmission input shaft, the rotational speed of the output shaft and the temperature of the pressure medium. For the case of failure of the electronic control device, the known device is provided with an emergency device by means of which a constant pressure ratio between the primary pulley and the secondary pulley is adjusted. Instead of a hydrodynamic converter between the prime mover and CVT, there can also be used a rigid mechanical connection or a mechanical connection damped by torsional vibration technology, by means of a shaft, torsional shock absorber, double solid fly wheel, etc. Since the reversing gear set is situated with positive engagement closing clutches for forward and reverse gears between the converter and the variator, no permanent connection of the variator with the engine exists in the described arrangement.
In another known CVT design, the variator is situated directly behind the torsional vibration damper of the prime mover, the positive engagement closing clutch lying upon the transmission output side. Therefore in this arrangement, a permanent mechanical connection of the variator with the engine does exist which can have a detrimental effect in case of a mechanical damage of the transmission.
The problem to be solved by this invention is to provide a method for monitoring the system pressure of a continuously variable transmission for motor vehicles which has a permanent mechanical coupling between engine and transmission and which, in the occurrence of mechanical damage and drop of system pressure determined thereby, takes care of switching off the engine while, at the same time, the vehicle is under the driver""s command.
On the basis of a method of the kind mentioned in detail above, this problem is solved.
The inventive method consists in that the system pressure in the CVT is continuously monitored, that upon detection of a system pressure drop in the CVT, a warning signal for the driver is activated and an entry is made in the transmission fault memory that the clutch on the output side is open, that by an electric connection of the electronic engine control, a signal is communicated for preventing an uncontrolled acceleration of the vehicle and that the vehicle speed is monitored until it falls below a preset threshold value after which the engine is switched off.
The inventive method therefore prevents the occurrence of capital secondary damages in the transmission in case of drop of the system pressure due to simple mechanical damages during an inadmissibly long operation. The method is implemented by the electronic transmission control which communicates with the electronic engine control, via an electric connection for example, via cable or the data bus (CAN coupling).
After a system pressure drop has been detected, the flow of power between engine and output line is interrupted and the CVT is uncoupled from the gears. By the simultaneous communication to the electronic engine control, the otherwise possible uncontrolled acceleration of the engine can be eliminated. After opening the clutch on the output side, the engine must be switched off as soon as possible. The assessment of the vehicle speed upon detection of the system pressure drop ensures the control of the vehicle by the driver by maintaining the engine support of brake and steering system.
The inventive method is also adequate for other mechanical errors in the CVT during which, after detection of a system pressure drop, a further rotation of the transmission inner parts must be prevented.